Remove ThreatByRank
This is a functional simplification that removes ThreatByRank.
STC
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 48009 W: 10630 L: 10560 D: 26819
http://tests.stockfishchess.org/tests/view/5d92095c0ebc594fb88eb61e
LTC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 18682 W: 3177 L: 3053 D: 12452
http://tests.stockfishchess.org/tests/view/5d9231120ebc594fb88ebacd
Moving forward, it's possible that ThreatByMinor and ThreatByRook
could be combined, but I haven't really contemplated that yet.
Closes https://github.com/official-stockfish/Stockfish/pull/2336
bench 4088701